1. Field of the Invention
The invention relates to a thermocyclic process having a short cycle time, typically a cycle time of less 30 minutes, especially a PSA (Pressure Swing Adsorption) process, using agglomerates that contain phase change materials (PCMs), so as to reduce the thermal effects that said thermocyclic process is subjected to during each cycle.
2. Related Art
The expression “thermocyclic process” refers to any cyclic process for which certain steps are exothermic, that is to say that are accompanied by a release of heat, whereas certain other steps are endothermic, that is to say are accompanied by a consumption of heat.
Typical examples of thermocyclic processes according to the present invention include:                processes for gas separation by pressure swing adsorption such as the PSA (Pressure Swing Adsorption) process, the VSA (Vacuum Swing Adsorption) process, the VPSA (Vacuum Pressure Swing Adsorption) process and the MPSA (Mixed Pressure Swing Adsorption) process,        any process that uses a chemical conversion coupled with pressure swing adsorption cycles as mentioned above, making it possible to shift the equilibrium of the chemical reactions.        
The processes for separation via pressure swing adsorption rely on the phenomenon of physical adsorption and make it possible to separate or purify gases by pressure cycling of the gas to be treated through one or more adsorbent beds, such as a zeolite, activated carbon, activated alumina, silica gel, molecular sieve or similar bed.
Within the context of the present invention, the term “PSA process” is understood to mean, unless stated otherwise, any process for gas separation via pressure swing adsorption, using a cyclic variation of the pressure between a high pressure, known as the adsorption pressure, and a low pressure, known as the regeneration pressure. Consequently, the generic name PSA process is used equally to denote the following cyclic processes:                VSA processes in which the adsorption is carried out substantially at atmospheric pressure, referred as “high pressure”, that is to say between 1 bara and 1.6 bara (bara=bar absolute), preferably between 1.1 and 1.5 bara, and the desorption pressure, referred to as “low pressure” is below atmospheric pressure, typically between 30 and 800 mbara, preferably between 100 and 600 mbara;        VPSA or MPSA processes in which the adsorption is carried out at a high pressure substantially greater than atmospheric pressure, generally between 1.6 and 8 bara, preferably between 2 and 6 bara, and the low pressure is below atmospheric pressure, typically between 30 and 800 mbara, preferably between 100 and 600 mbara; and        PSA processes in which the adsorption is carried out at a high pressure significantly above atmospheric pressure, typically between 1.6 and 50 bara, preferably between 2 and 35 bara, and the low pressure is greater than or substantially equal to atmospheric pressure, therefore between 1 and 9 bara, preferably between 1.2 and 2.5 bara.        
Mention will subsequently be made of a “RPSA process” to denote PSA processes having a very rapid cycle, in general less than one minute.
Generally, a PSA process makes it possible to separate one or more gas molecules from a gas mixture containing them, by exploiting the difference in affinity of a given adsorbent or, if necessary, of several adsorbents for these various gas molecules.
The affinity of an adsorbent for a gas molecule depends on the structure and on the composition of the adsorbent, and also on the properties of the molecule, especially its size, its electronic structure and its multipole moments.
An adsorbent may be, for example, a zeolite, an activated carbon, an activated alumina, a silica gel, a carbon-based or non-carbon-based molecule sieve, a metallorganic structure, one or more oxides or hydroxides of alkali or alkaline-earth metals, or a porous structure containing a substance capable of reacting reversibly with one or more gas molecules, such as amines, physical solvents, metallic complexing agents, metal oxides or hydroxides, for example.
Adsorption is an exothermic phenomenon, each molecule-adsorbent pair being characterized by an isosteric adsorption enthalpy or a reaction enthalpy in general. Symmetrically, desorption is endothermic.
Furthermore, a PSA process is a cyclic process comprising several sequential steps of adsorption and desorption.
Consequently, certain steps of the cycle of a PSA are exothermic, in particular the step of adsorption of the gas molecules adsorbed on the adsorbent, whereas other steps are endothermic, in particular the step of regeneration or desorption of the molecules adsorbed on the adsorbent.
The thermal effects that result from the adsorption enthalpy or from the reaction enthalpy lead, generally, to the propagation, for each cycle, of an adsorption heat wave limiting the adsorption capacities and a desorption cold wave limiting the desorption.
This local cyclic phenomenon of temperature fluctuations has a sizeable impact on the separation performances of the process, such as the productivity, the separation efficiency and the specific separation energy, as mentioned in document EP-A-1 188 470.
Thus, it has been shown that if the thermal fluctuations due to the adsorption enthalpy were completely eradicated, the productivity of certain current industrial O2 PSAs would be improved by around 50% and the oxygen yield would be improved by 10%. Similarly, for other types of PSA, the attenuation of the thermal fluctuations would lead to a significant improvement in the separation performances.
Since this negative phenomenon was identified, several solutions have always been described in order to attempt to reduce it or eliminate it.
Thus, it has been proposed to increase the heat capacity of the adsorbent medium by addition of an inert binder, during the manufacture of the particles, by deposition of the adsorbent medium onto an inert core, by addition of particles that are identical to the adsorbent but inert. For example, in the case of an O2 PSA process, the following has already been tested: adsorbing the nitrogen contained in the air onto a composite bed composed of zeolites 5A and 3A, which are only differentiated by the size of their pores: only those of zeolite 5A allow the adsorption of nitrogen, since those of zeolite 3A are too small in size.
Furthermore, the use of outside heating and/or cooling means has also been described for counter-balancing the thermal effects of the desorption or of the adsorption, such as the use of heat exchangers.
Thermal couplings between the adsorption and the regeneration phase has also been proposed, the adsorbent being positioned in the successive passages of a plate heat exchanger, the circulation of fluids then being organized so that the passages are alternatively in the adsorption phase and desorption phase.
Another solution that makes it possible to reduce the amplitude of the thermal fluctuations consists in adding to the adsorbent bed a phase change material (PCM) as described by document U.S. Pat. No. 4,971,605. In this way the heat of adsorption and of desorption, or some of this heat, is adsorbed in the form of latent heat by the PCM, at the temperature, or in the temperature range, of the phase change of the PCM. It is then possible to operate the PSA unit in a mode closer to isothermal.
In practice, the phase change materials (PCMs) act as heat sinks at their phase change temperature, or over their phase change temperature range between a lower phase change temperature and an upper phase change temperature.
PCMs may be organic, such as paraffins, fatty acids, nitrogen-containing compounds, oxygen-containing compounds (alcohol or acids), phenyls and silicones, or inorganic such as hydrated salts and metal alloys. The term PCM will be used to refer to one of these compounds in the pure state or any mixture containing one of these compounds (such as, for example, eutectic mixtures).
The heat adsorption capacity of a PCM is even greater when its latent heat is high. Generally, PCMs are used for their solid-liquid phase change.
In order to be able to handle the PCMs, whether they are in the solid or liquid state, they may be micro-encapsulated in a micron-sized solid shell, preferably based on polymers (melamine formaldehyde, acrylic, etc.).
Since paraffins in particular are relatively easy to microencapsulate, they are generally the PCMs of choice compared to hydrated salts, even if the paraffins have a latent heat generally lower than those of hydrated salts.
Furthermore, paraffins have other advantages such as the reversibility of the phase change, chemical stability, defined phase change temperature or defined lower and upper phase change temperatures (that is to say that there is no hysteresis effect), a low cost, limited toxicity and a wide choice of phase change temperatures depending on the number of carbon atoms and the structure of the molecule.
Microencapsulated paraffinic PCMs are in the form of a powder, each microcapsule constituting this powder being between 50 nm and 100 μm in diameter, preferably between 0.2 and 50 μm in diameter. Each microcapsule has a thermal conductivity of around 0.1 to 0.2 W/(m·K), depending on whether the paraffin is in the solid or liquid state inside the microcapsule.
Microencapsulated PCMs, available in powder form, cannot be introduced as is into an adsorbent bed since they would be carried along by the gas streams circulating in the adsorber.
Document EP-A-1 565 539 describes various ways of placing these microcapsules in the immediate vicinity of the adsorbent, namely one of the materials at the side, the surface or inside the other, so that they can play their part in storing/withdrawing the heat flows linked respectively to the adsorption and to the desorption.
However, the solutions described in this document cannot, or can only with difficulty, be applied industrially.
One problem that is faced then is in being able to use microencapsulated PCMs in an industrial PSA process for separation of gases by adsorption, in particular in a PSA process having a short cycle time, that is to say less than or equal to 30 minutes.